Membraneless seawater desalination

ABSTRACT

Disclosed are microfluidic devices and systems for the desalination of water. The devices and systems can include an electrode configured to generate an electric field gradient in proximity to an intersection formed by the divergence of two microfluidic channels from an inlet channel. Under an applied bias and in the presence of a pressure driven flow of saltwater, the electric field gradient can preferentially direct ions in saltwater into one of the diverging microfluidic channels, while desalted water flows into second diverging channel. Also provided are methods of using the devices and systems described herein to decrease the salinity of water.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application No.61/740,780, filed Dec. 21, 2012, which is hereby incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government Support under AgreementDE-FG02-06ER15758 awarded by the U.S. Department of Energy, and ContractEP-D-12-026 awarded by the U.S. Environmental Protection Agency. TheGovernment has certain rights in the invention.

TECHNICAL FIELD

This application relates generally to devices, systems, and methods forthe desalination of water.

BACKGROUND

The global demand for freshwater is growing rapidly. Many conventionalsources of freshwater, including lakes, rivers, and aquifers, arerapidly becoming depleted. As a consequence, freshwater is becoming alimited resource in many regions. In fact, the United Nations estimatestwo-thirds of the world's population could be living in water stressedregions by 2025.

Currently, approximately 97% of the world's water supply is present asseawater. Desalination—the process by which salinated water (e.g.,seawater) is converted to fresh water—offers the potential to providedependable supplies of freshwater suitable for human consumption orirrigation. Unfortunately, existing desalination processes, includingdistillation and reverse osmosis, require both large amounts of energyand specialized, expensive infrastructure. As a consequence,desalination is currently expensive compared to most conventionalsources of water, and often prohibitively expensive in developingregions of the world. Therefore, only a small fraction of total humanwater use is currently satisfied by desalination. More energy efficientmethods for water desalination offer the potential to address theincreasing demands for freshwater, particularly in water stressedregions.

SUMMARY

Disclosed are microfluidic devices and systems for the desalination ofwater.

Microfluidic devices for the desalination of water can comprise adesalination unit. The desalination unit can comprise an inlet channelfluidly connected to a dilute outlet channel and a concentrated outletchannel. The dilute outlet channel and the concentrated outlet channelcan diverge from the inlet channel at an intersection. The desalinationunit can further comprise an electrode in electrochemical contact withthe desalination unit. The electrode can be configured to generate anelectric field gradient in proximity to the intersection where diluteoutlet channel and concentrated outlet channel diverge from the inletchannel. Under an applied bias and in the presence of a flow ofsaltwater, the electric field gradient can preferentially direct ions inthe saltwater into concentrated outlet channel, while desalted waterflows into the dilute outlet channel.

In some embodiments, the microfluidic device can further include anauxiliary channel fluidly isolated from the desalination unit. Theauxiliary channel can be electrochemically connected to the desalinationunit via a bipolar electrode. In these cases, the bipolar electrode canbe configured to be in electrochemical contact with both thedesalination unit and the auxiliary channel. Under an applied biasacross the auxiliary channel and the desalination unit and in thepresence of a flow of saltwater, the electric field gradient canpreferentially direct ions in the saltwater into concentrated outletchannel of the desalination unit, while desalted water flows into thedilute outlet channel.

In some embodiments, the auxiliary channel comprises a desalinationunit. In these embodiments, the microfluidic device can comprise twodesalination units, which can be of identical or different structure.The first desalination unit can be electrochemically connected to thesecond desalination unit by a bipolar electrode. Under an applied biasacross the first desalination unit and the second desalination unit andin the presence of a pressure driven flow of saltwater, the electricfield gradient can preferentially direct ions in the saltwater intoconcentrated outlet channels of the first and second desalination units,while desalted water flows into the dilute outlet channels of the firstand second desalination units.

A plurality of the microfluidic devices described herein can be combinedto form a water purification system. The system can comprise a pluralityof the devices described herein arranged in parallel or fluidlyconnected in series. The systems can also comprise a plurality ofdevices both arranged in parallel and fluidly connected in series. Forexample, the device can include a first pair of devices fluidlyconnected in series which are arranged in parallel with a second pair ofdevices fluidly connected in series. In such systems, the plurality ofdevices can be fabricated in a single plane (i.e., as a 2-dimensionalsystem) or in three dimensions.

Also provided are methods of using the devices and systems describedherein to decrease the salinity of water.

DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic drawing illustrating a microfluidic device forthe desalination of water.

FIG. 1B is a schematic drawing illustrating an enlarged portion of themicrofluidic device shown in FIG. 1A.

FIG. 1C is a schematic drawing illustrating a microfluidic device forthe desalination of water in combination with a power supply configuredto apply a potential bias across the desalination unit.

FIG. 2 is a schematic drawing illustrating a microfluidic device for thedesalination of water. The device includes a desalination unit and anauxiliary channel electrochemically connected by a bipolar electrode.

FIG. 3 is a schematic drawing illustrating a microfluidic device for thedesalination of water. The device includes two desalination unitselectrochemically connected by a bipolar electrode.

FIG. 4 is a schematic drawing of a water purification system for thedesalination of water. The system includes multiple desalination unitsconfigured to operate in parallel.

FIG. 5 is a schematic drawing of a water purification system for thedesalination of water. The system includes multiple desalination unitsconfigured to operate in parallel.

FIG. 6 is a schematic drawing of a water purification system for thedesalination of water. The device includes multiple desalination unitsconfigured to operate in series.

FIGS. 7A-7B are fluorescence micrographs illustrating the flow of asolution of Ru(bpy)²⁺ (a fluorescent cationic tracer) in saltwaterthrough the device illustrated in FIG. 2. FIG. 7A is a fluorescencemicrograph of the device taken before application of a potential bias.FIG. 7B is a fluorescence micrograph of the device taken uponapplication of a potential bias.

FIG. 8 is a fluorescence micrograph illustrating the flow of a solutionof Ru(bpy)²⁺ (a fluorescent cationic tracer) in Na₂SO₄ through thedevice illustrated in FIG. 2 upon application of a potential bias.

FIG. 9 is a graph of total current flowing through the deviceillustrated in FIG. 2 (i_(tot), plotted in nanoamperes) as a function oftime (in seconds) during operation.

DETAILED DESCRIPTION

Disclosed are microfluidic devices and systems for the desalination ofwater.

Microfluidic devices for the desalination of water can comprise adesalination unit. The desalination unit can comprise an inlet channelfluidly connected to a dilute outlet channel and a concentrated outletchannel. The dilute outlet channel and the concentrated outlet channelcan diverge from the inlet channel at an intersection. The desalinationunit can also comprise an electrode in electrochemical contact with thedesalination unit. The electrode can be configured to generate anelectric field gradient in proximity to the intersection where diluteoutlet channel and concentrated outlet channel diverge from the inletchannel.

An example device comprising a desalination unit (100) is schematicallyillustrated in FIG. 1A. The desalination unit includes an inlet channel(102) fluidly connected to a dilute outlet channel (104) and aconcentrated outlet channel (106). The dilute outlet channel (104) andthe concentrated outlet channel (106) diverge from the inlet channel(102) at an intersection (107). An electrode (108) is positioned inproximity to the intersection (107). The electrode (108) is configuredto form an ion depletion zone (109) at and downstream of the electrodeduring device operation, resulting in the formation of an electric fieldgradient in proximity to the intersection. The example device furtherincludes a fluid reservoir (110) fluidly connected to the upstreamterminus of the inlet channel (102), a fluid reservoir (114) fluidlyconnected to the downstream terminus of the dilute outlet channel (104),and a fluid reservoir (112) fluidly connected to the downstream terminusof the concentrated outlet channel (106).

The dimensions of the microfluidic channels in the desalination unit(100) (e.g., the inlet channel (102), the dilute outlet channel (104),and the concentrated outlet channel (106)) can individually and/or incombination be selected in view of a number of factors, including thesize and position of the electrode relative to the microfluidic channelsin the desalination unit, the desired device flow rate, salinity of thesaltwater being treated using the device, and the desired degree ofsalinity reduction.

In some instances, the dimensions of the inlet channel (102), the diluteoutlet channel (104), and the concentrated outlet channel (106) areselected such that the sum of the area of a cross-section of diluteoutlet channel and the area of a cross-section of the concentratedoutlet channel is substantially equal to the area of a cross-section ofthe inlet channel. In this context, substantially equal can mean thatthe sum of the area of a cross-section of dilute outlet channel and thearea of a cross-section of the concentrated outlet channel is with forexample, 15%, of the area of a cross-section of the inlet channel (e.g.,within 10% of the area of a cross-section of the inlet channel, orwithin 5% of the area of a cross-section of the inlet channel). In someembodiments, the dilute outlet channel, and the concentrated outletchannel have substantially equivalent cross-sectional dimensions,meaning that the height and width of the dilute outlet channel aresubstantially equivalent (e.g., within 15%, within 10%, or within 5%) tothe height and width of the concentrated outlet channel.

The dimensions of the microfluidic channels in the desalination unit(100) (e.g., the inlet channel (102), the dilute outlet channel (104),and the concentrated outlet channel (106)) can be fabricated so as tohave a variety of cross-sectional shapes. In some embodiments, themicrofluidic channels in the desalination unit (e.g., the inlet channel,the dilute outlet channel, and the concentrated outlet channel) have asubstantially square or rectangular cross-sectional shape.

In some embodiments, the inlet channel (102) has a width of about 1000microns or less (e.g., about 900 microns or less, about 800 microns orless, about 750 microns or less, about 700 microns or less, about 600microns or less, about 500 microns or less, about 400 microns or less,about 300 microns or less, about 250 microns or less, about 200 micronsor less, about 150 microns or less, about 100 microns or less, about 75microns or less, or about 50 microns or less). In some embodiments, theinlet channel (102) has a width of at least about 1 micron (e.g., atleast about 5 microns, at least about 10 microns, at least about 15microns, at least about 20 microns, at least about 25 microns, at leastabout 50 microns, at least about 75 microns, at least about 100 microns,at least about 150 microns, at least about 200 microns, at least about250 microns, at least about 300 microns, at least about 400 microns, atleast about 500 microns, at least about 600 microns, at least about 700microns, at least about 750 microns, at least about 800 microns, atleast about 900 microns, or at least about 1000 microns).

The inlet channel (102) can have a width that ranges from any of theminimum dimensions to any of the maximum dimensions described above. Forexample, the inlet channel (102) can have a width that ranges from about1000 microns to about 1 micron (e.g., from about 750 microns to about 5microns, from about 500 microns to about 10 microns, from about 250microns to about 20 microns, or from about 150 microns to about 25microns).

In some embodiments, the inlet channel (102) has a height of about 50microns or less (e.g., about 45 microns or less, about 40 microns orless, about 35 microns or less, about 30 microns or less, about 25microns or less, about 20 microns or less, about 15 microns or less,about 10 microns or less, about 9 microns or less, about 8 microns orless, about 7.5 microns or less, about 7 microns or less, about 6microns or less, about 5 microns or less, about 4 microns or less, about3 microns or less, about 2.5 microns or less, or about 2 microns orless). In some embodiments, the inlet channel (102) has a height of atleast about 1 micron (e.g., at least about 2 microns, at least about 2.5microns, at least about 3 microns, at least about 4 microns, at leastabout 5 microns, at least about 6 microns, at least about 7 microns, atleast about 7.5 microns, at least about 8 microns, at least about 9microns, at least about 10 microns, at least about 15 microns, at leastabout 20 microns, at least about 25 microns, at least about 30 microns,at least about 35 microns, at least about 40 microns, or at least about45 microns).

The inlet channel (102) can have a height that ranges from any of theminimum dimensions to any of the maximum dimensions described above. Forexample, the inlet channel (102) can have a height that ranges fromabout 50 microns to about 1 micron (e.g., from about 45 microns to about1 micron, from about 40 microns to about 1 micron, from about 35 micronsto about 1 micron, from about 30 microns to about 1 micron, from about25 microns to about 1 micron, or from about 20 microns to about 1micron).

In some embodiments, the dilute outlet channel (104) has a width ofabout 500 microns or less (e.g., about 450 microns or less, about 400microns or less, about 350 microns or less, about 300 microns or less,about 250 microns or less, about 200 microns or less, about 150 micronsor less, about 125 microns or less, about 100 microns or less, about 75microns or less, about 50 microns or less, about 25 microns or less,about 20 microns or less, about 15 microns or less, about 10 microns orless, about 5 microns or less, or about 1 micron or less). In someembodiments, the dilute outlet channel (104) has a width of at leastabout 0.5 microns (e.g., at least about 1 micron, at least about 2.5microns, at least about 5 microns, at least about 10 microns, at leastabout 15 microns, at least about 20 microns, at least about 25 microns,at least about 50 microns, at least about 75 microns, at least about 100microns, at least about 150 microns, at least about 200 microns, atleast about 250 microns, at least about 300 microns, at least about 400microns, or at least about 450 microns).

The dilute outlet channel (104) can have a width that ranges from any ofthe minimum dimensions to any of the maximum dimensions described above.For example, the dilute outlet channel (104) can have a width thatranges from about 500 microns to about 0.5 microns (e.g., from about 400microns to about 1 micron, from about 250 microns to about 1 micron,from about 150 microns to about 5 microns, or from about 80 microns toabout 10 microns).

In some embodiments, the dilute outlet channel (104) has a height ofabout 50 microns or less (e.g., about 45 microns or less, about 40microns or less, about 35 microns or less, about 30 microns or less,about 25 microns or less, about 20 microns or less, about 15 microns orless, about 10 microns or less, about 9 microns or less, about 8 micronsor less, about 7.5 microns or less, about 7 microns or less, about 6microns or less, about 5 microns or less, about 4 microns or less, about3 microns or less, about 2.5 microns or less, or about 2 microns orless). In some embodiments, the dilute outlet channel (104) has a heightof at least about 1 micron (e.g., at least about 2 microns, at leastabout 2.5 microns, at least about 3 microns, at least about 4 microns,at least about 5 microns, at least about 6 microns, at least about 7microns, at least about 7.5 microns, at least about 8 microns, at leastabout 9 microns, at least about 10 microns, at least about 15 microns,at least about 20 microns, at least about 25 microns, at least about 30microns, at least about 35 microns, at least about 40 microns, or atleast about 45 microns).

The dilute outlet channel (104) can have a height that ranges from anyof the minimum dimensions to any of the maximum dimensions describedabove. For example, the dilute outlet channel (104) can have a heightthat ranges from about 50 microns to about 1 micron (e.g., from about 45microns to about 1 micron, from about 40 microns to about 1 micron, fromabout 35 microns to about 1 micron, from about 30 microns to about 1micron, from about 25 microns to about 1 micron, or from about 20microns to about 1 micron).

In some embodiments, the concentrated outlet channel (106) has a widthof about 500 microns or less (e.g., about 450 microns or less, about 400microns or less, about 350 microns or less, about 300 microns or less,about 250 microns or less, about 200 microns or less, about 150 micronsor less, about 125 microns or less, about 100 microns or less, about 75microns or less, about 50 microns or less, about 25 microns or less,about 20 microns or less, about 15 microns or less, about 10 microns orless, about 5 microns or less, or about 1 micron or less). In someembodiments, the concentrated outlet channel (106) has a width of atleast about 0.5 microns (e.g., at least about 1 micron, at least about2.5 microns, at least about 5 microns, at least about 10 microns, atleast about 15 microns, at least about 20 microns, at least about 25microns, at least about 50 microns, at least about 75 microns, at leastabout 100 microns, at least about 150 microns, at least about 200microns, at least about 250 microns, at least about 300 microns, atleast about 400 microns, or at least about 450 microns).

The concentrated outlet channel (106) can have a width that ranges fromany of the minimum dimensions to any of the maximum dimensions describedabove. For example, the concentrated outlet channel (106) can have awidth that ranges from about 500 microns to about 0.5 microns (e.g.,from about 400 microns to about 1 micron, from about 250 microns toabout 1 micron, from about 150 microns to about 5 microns, or from about80 microns to about 10 microns).

In some embodiments, the concentrated outlet channel (106) has a heightof about 50 microns or less (e.g., about 45 microns or less, about 40microns or less, about 35 microns or less, about 30 microns or less,about 25 microns or less, about 20 microns or less, about 15 microns orless, about 10 microns or less, about 9 microns or less, about 8 micronsor less, about 7.5 microns or less, about 7 microns or less, about 6microns or less, about 5 microns or less, about 4 microns or less, about3 microns or less, about 2.5 microns or less, or about 2 microns orless). In some embodiments, the concentrated outlet channel (106) has aheight of at least about 1 micron (e.g., at least about 2 microns, atleast about 2.5 microns, at least about 3 microns, at least about 4microns, at least about 5 microns, at least about 6 microns, at leastabout 7 microns, at least about 7.5 microns, at least about 8 microns,at least about 9 microns, at least about 10 microns, at least about 15microns, at least about 20 microns, at least about 25 microns, at leastabout 30 microns, at least about 35 microns, at least about 40 microns,or at least about 45 microns).

The concentrated outlet channel (106) can have a height that ranges fromany of the minimum dimensions to any of the maximum dimensions describedabove. For example, the concentrated outlet channel (106) can have aheight that ranges from about 50 microns to about 1 micron (e.g., fromabout 45 microns to about 1 micron, from about 40 microns to about 1micron, from about 35 microns to about 1 micron, from about 30 micronsto about 1 micron, from about 25 microns to about 1 micron, or fromabout 20 microns to about 1 micron).

The length of the microfluidic channels in the desalination unit (100)(e.g., the inlet channel (102), the dilute outlet channel (104), and theconcentrated outlet channel (106)) can vary. The length of themicrofluidic channels in the desalination unit can individually beselected in view of a number of the overall device design and otheroperational considerations. In some embodiments, the inlet channel(102), the dilute outlet channel (104), and the concentrated outletchannel (106) each have a length of at least about 0.1 cm (e.g., atleast about 0.2 cm, at least about 0.3 cm, at least about 0.4 cm, atleast about 0.5 cm, at least about 0.6 cm, at least about 0.7 cm, atleast about 0.8 cm, at least about 0.9 cm, at least about 1 cm, at leastabout 2 cm, at least about 2.5 cm, at least about 3 cm, at least about 4cm, at least about 5 cm, or longer). The microfluidic channels in thedesalination unit can be substantially linear in shape, or they canpossess one or more non-linear regions (e.g., a curved region, a spiralregion, an angular region, or combinations thereof) along the length oftheir fluid flow path.

With reference again to FIG. 1A, the dilute outlet channel (104) and theconcentrated outlet channel (106) diverge from the inlet channel (102)at an intersection (107). The orientation of the dilute outlet channel(104) and the concentrated outlet channel (106) with respect to oneanother at the intersection can be varied. The angle formed between thedilute outlet channel (104) and the concentrated outlet channel (106) ina device can be selected in view of a number of parameters, includingthe size and position of the electrode relative to the microfluidicchannels in the desalination unit, the desired device flow rate,salinity of the saltwater being treated using the device, and thedesired degree of salinity reduction.

In some cases, the angle formed between the dilute outlet channel (104)and the concentrated outlet channel (106) at the intersection (107) isabout 60 degrees or less (e.g., about 55 degrees or less, about 50degrees or less, about 45 degrees or less, about 40 degrees or less,about 35 degrees or less, about 30 degrees or less, about 25 degrees orless, about 20 degrees or less, about 15 degrees or less, or less).

The electrode (108) can be fabricated from any suitable conductivematerial, such as a metal (e.g., gold), metal alloy, metal oxide, orconductive carbon. The electrode (108) is configured so as to be inelectrochemical contact with the desalination unit (100), meaning thatthe electrode (108) can participate in a faradaic reaction with one ormore components of a solution present in a microfluidic channel of thedesalination unit. For example, the electrode (108) can be configuredsuch that a surface of the electrode is in direct contact with fluidpresent in a microfluidic channel of the desalination unit. The devicecan be configured such that the electrode (108) can function as eitheran anode, cathode, or anode and cathode during device operation.

The position and dimensions of the electrode (108) relative to thedesalination unit can be selected in view of a number of factors,including the size and configuration of the microfluidic channels in thedesalination unit, the desired device flow rate, salinity of thesaltwater being treated using the device, and the desired degree ofsalinity reduction. The electrode (108) can have a variety of2-dimensional or 3-dimensional shapes, provided that the electrode (108)can be integrated into the device, and is compatible with the formationof an electric field gradient suitable to direct ions flowing throughthe inlet channel (102) preferentially into the concentrated outletchannel (106). In certain embodiments, the electrode (108) is aconductive surface (e.g., a line, a rectangular pad, or a square pad)substantially co-planar with the floor of the inlet channel (102), andintegrated into the floor of the inlet channel in proximity to theintersection (107). In other embodiments, the electrode (108) is aconductive surface (e.g., a line, a rectangular pad, or a square pad)that is fabricated onto/into the floor of the inlet channel in proximityto the intersection (107), and which extends from the floor of the inletchannel into the inlet channel. In these embodiments, the electrode canbe said to have a height, measured as the distance from the floor of theinlet channel to the surface or edge of the electrode within the inletchannel positioned at greatest distance from the floor of the inletchannel.

With reference again to FIG. 1A, the electrode (108) can be positionedin proximity to the intersection (107) so as to form an ion depletionzone (109) at and downstream of the electrode (108), and extending intothe dilute outlet channel (104) during device operation. The iondepletion zone (109) can optionally extend into a portion of theconcentrated inlet channel (106). In some embodiments, the electrode(108) is positioned within the floor of the inlet channel (102) upstreamof the opening of the dilute outlet channel (104).

By way of exemplification, FIG. 1B illustrates an enlarged view of theintersection (107) of the device shown in FIG. 1A. The electrode (108)is positioned within the floor of the inlet channel (102). The surfaceof the electrode (108) in electrochemical contact with the desalinationunit is positioned approximately ±50 microns (measured as the distancefrom the opening of the dilute outlet channel to the downstream edge ofthe electrode, 130) upstream or downstream of the opening of the diluteoutlet channel (104).

In certain embodiments, the surface of the electrode (108) inelectrochemical contact with the desalination unit is positionedupstream of the opening of the dilute outlet channel (104), and withinabout 500 microns of the opening of the dilute outlet channel (e.g.,within about 400 microns, within about 300 microns, within about 250microns, within about 200 microns, within about 150 microns, withinabout 100 microns, within about 90 microns, within about 80 microns,within about 75 microns, within about 70 microns, within about 60microns, within about 50 microns, within about 40 microns, within about30 microns, within about 25 microns, within about 20 microns, or withinabout 10 microns).

In some embodiments, the surface of the electrode (108) inelectrochemical contact with the desalination unit is positioneddownstream of the opening of the dilute outlet channel (104), and withinabout 100 microns of the opening of the dilute outlet channel (e.g.,within about 90 microns, within about 80 microns, within about 75microns, within about 70 microns, within about 60 microns, within about50 microns, within about 40 microns, within about 30 microns, withinabout 25 microns, within about 20 microns, within about 10 microns, orwithin about 5 microns). When the surface of the electrode (108) inelectrochemical contact with the desalination unit is positioneddownstream of the opening of the dilute outlet channel (104), the lengthof the electrode (as discussed below) must be sufficient such that atleast a portion of the electrode (108) in electrochemical contact withthe desalination unit extends beyond the opening of the dilute outletchannel (104), and into the inlet channel (i.e., a portion of theelectrode must be located upstream of the dilute outlet channel)

Again referring to FIG. 1B, the surface of the electrode (108) inelectrochemical contact with the desalination unit can have a width(132, measured as the distance from one side of the surface of theelectrode to the other side of the surface of the electrode along anaxis perpendicular to the direction of fluid flow through the inletchannel) and a length (134, measured as the distance from one side ofthe surface of the electrode to the other side of the surface of theelectrode along an axis parallel to the direction of fluid flow throughthe inlet channel). By way of exemplification, in the example device toFIG. 1B, the surface of the electrode (108) in electrochemical contactwith the desalination unit has a width (132) that is about equal to thewidth of the dilute outlet channel (104) (50 microns), and a length(134) of about 100 microns.

In some embodiments, the surface of the electrode (108) inelectrochemical contact with the desalination unit has a width (132) ofat least about 50% of the width of the dilute outlet channel (104)(e.g., at least about 60% of the width of the dilute outlet channel, atleast about 70% of the width of the dilute outlet channel, at leastabout 75% of the width of the dilute outlet channel, at least about 80%of the width of the dilute outlet channel, at least about 90% of thewidth of the dilute outlet channel, at least about 90% of the width ofthe dilute outlet channel, at least the width of the dilute outletchannel, at least about 105% of the width of the dilute outlet channel,or at least about 110% of the width of the dilute outlet channel). Insome embodiments, the surface of the electrode (108) in electrochemicalcontact with the desalination unit has a width (132) that is less thanabout 150% of the width of the dilute outlet channel (104) (e.g., lessthan about 140% of the width of the dilute outlet channel, less thanabout 130% of the width of the dilute outlet channel, less than about125% of the width of the dilute outlet channel, less than about 120% ofthe width of the dilute outlet channel, less than about 110% of thewidth of the dilute outlet channel, less than about 105% of the width ofthe dilute outlet channel, or less than the width of the dilute outletchannel).

The surface of the electrode (108) in electrochemical contact with thedesalination unit can have a width (132) that ranges from any of theminimum dimensions to any of the maximum dimensions described above. Forexample, the surface of the electrode (108) in electrochemical contactwith the desalination unit can have a width (132) that ranges from about50% of the width of the dilute outlet channel (104) to about 150% of thewidth of the dilute outlet channel (e.g., from about 75% of the width ofthe dilute outlet channel to about 125% of the width of the diluteoutlet channel, from about 90% of the width of the dilute outlet channelto about 110% of the width of the dilute outlet channel, or from about95% of the width of the dilute outlet channel to about 105% of the widthof the dilute outlet channel). In certain embodiments, the surface ofthe electrode (108) in electrochemical contact with the desalinationunit has a width (132) that is about equal to the width of the diluteoutlet channel (104).

In some embodiments, the surface of the electrode (108) inelectrochemical contact with the desalination unit has a width (132)that is at least about 25% of the width of the inlet channel (102)(e.g., at least about 30% of the width of the inlet channel, at leastabout 40% of the width of the inlet channel, at least about 45% of thewidth of the inlet channel, at least about 50% of the width of the inletchannel, at least about 55% of the width of the inlet channel, or atleast about 60% of the width of the inlet channel). In some embodiments,the surface of the electrode (108) in electrochemical contact with thedesalination unit has a width (132) that is less than about 75% of thewidth of the inlet channel (102) (e.g., less than about 60% of the widthof the inlet channel, less than about 55% of the width of the inletchannel, less than about 50% of the width of the inlet channel, lessthan about 45% of the width of the inlet channel, or less than about 40%of the width of the inlet channel).

The surface of the electrode (108) in electrochemical contact with thedesalination unit can have a width (132) that ranges from any of theminimum dimensions to any of the maximum dimensions described above. Forexample, the surface of the electrode (108) in electrochemical contactwith the desalination unit can have a width (132) that ranges from about25% of the width of the inlet channel (102) to about 75% of the width ofthe inlet channel (e.g., from about 30% of the width of the diluteoutlet channel to about 70% of the width of the dilute outlet channel,from about 40% of the width of the dilute outlet channel to about 60% ofthe width of the dilute outlet channel, or from about 45% of the widthof the dilute outlet channel to about 55% of the width of the diluteoutlet channel). In certain embodiments, the surface of the electrode(108) in electrochemical contact with the desalination unit has a width(132) that is about 50% of the width of the inlet channel (102).

In some embodiments, the surface of the electrode (108) inelectrochemical contact with the desalination unit has a width (132) ofabout 600 microns or less (e.g., about 500 microns or less, about 450microns or less, about 400 microns or less, about 350 microns or less,about 300 microns or less, about 250 microns or less, about 200 micronsor less, about 150 microns or less, about 125 microns or less, about 100microns or less, about 75 microns or less, about 50 microns or less,about 25 microns or less, about 20 microns or less, about 15 microns orless, about 10 microns or less, about 5 microns or less, or about 1micron or less). In some embodiments, the surface of the electrode (108)in electrochemical contact with the desalination unit has a width (132)of at least about 0.5 microns (e.g., at least about 1 micron, at leastabout 2.5 microns, at least about 5 microns, at least about 10 microns,at least about 15 microns, at least about 20 microns, at least about 25microns, at least about 50 microns, at least about 75 microns, at leastabout 100 microns, at least about 150 microns, at least about 200microns, at least about 250 microns, at least about 300 microns, atleast about 400 microns, at least about 450 microns, or at least about500 microns).

The surface of the electrode (108) in electrochemical contact with thedesalination unit can have a width (132) that ranges from any of theminimum dimensions to any of the maximum dimensions described above. Forexample, the surface of the electrode (108) in electrochemical contactwith the desalination unit can have a width (132) that ranges from about600 microns to about 0.5 microns (e.g., from about 400 microns to about1 micron, from about 250 microns to about 1 micron, from about 150microns to about 5 microns, or from about 80 microns to about 10microns).

The length (134) of the surface of the electrode (108) inelectrochemical contact with the desalination unit can be varied. Insome embodiments the surface of the electrode (108) has a length (134)of at least about 10 microns (e.g., at least about 15 microns, at leastabout 20 microns, at least about 25 microns, at least about 50 microns,at least about 75 microns, at least about 100 microns, at least about150 microns, at least about 200 microns, at least about 250 microns, atleast about 300 microns, at least about 400 microns, at least about 450microns, or at least about 450 microns). In some embodiments, thesurface of the electrode (108) has a length (134) of less than about 500microns (e.g., less than about 400 microns, less than about 300 microns,less than about 250 microns, less than about 200 microns, or less thanabout 100 microns).

The surface of the electrode (108) in electrochemical contact with thedesalination unit can have a length (134) that ranges from any of theminimum dimensions to any of the maximum dimensions described above. Forexample, the surface of the electrode (108) can have a length (134) thatranges from about 10 microns to about 500 microns (e.g., from about 25microns to about 250 microns, or from about 50 microns to about 150microns).

The height of the electrode (108) in electrochemical contact with thedesalination unit can also be varied. The height of the electrode (108)can be selected in view of a number of factors, including the height ofthe microfluidic channels in the desalination unit. In some cases, theheight of the electrode (108) is approximately zero (i.e., the electrodeis substantially co-planar with the floor of the inlet channel). In someembodiments, the height of the electrode (108) is less than about 1micron (e.g., less than about 900 nm, less than about 800 nm, less thanabout 750 nm, less than about 700 nm, less than about 600 nm, less thanabout 500 nm, less than about 400 nm, less than about 300 nm, less thanabout 250 nm, less than about 200 nm, or less than about 100 nm).

As shown in FIG. 1C, a power supply (140) can be configured to apply apotential bias across the desalination unit. A flow of saltwater (120)can be initiated from the inlet channel (102) to the dilute outletchannel (104) and the concentrated outlet channel (106). Uponapplication of a potential bias, an ion depletion zone (109) andsubsequent electric field gradient are formed near the electrode (108)in proximity to the intersection (107). As a consequence, ions in thesaltwater are preferentially directed into the concentrated outletchannel (106), resulting in a brine (122) flowing through theconcentrated outlet channel. Desalted water (i.e., water containing lesssalt that the saltwater introduced into the inlet channel; 124) flowsinto the dilute outlet channel (104).

In some embodiments, the microfluidic device can further include anauxiliary channel fluidly isolated from the desalination unit. Anexample device comprising a desalination unit (100) and an auxiliarychannel (202) is schematically illustrated in FIG. 2. The desalinationunit includes an inlet channel (102) fluidly connected to a diluteoutlet channel (104) and a concentrated outlet channel (106). The diluteoutlet channel (104) and the concentrated outlet channel (106) divergefrom the inlet channel (102) at an intersection (107). The device alsoincludes and an auxiliary channel (202) which is fluidly isolated fromthe desalination unit (100).

The auxiliary channel (202) can comprise, for example, a singlemicrofluidic channel. In these embodiments, dimensions of the auxiliarychannel (e.g., height, width, and length) can vary. The dimensions ofthe auxiliary channel (202) can individually be selected in view of anumber of the overall device design and other operationalconsiderations. The auxiliary channel (202) can be substantially linearin shape, or it can possess one or more non-linear regions (e.g., acurved region, a spiral region, an angular region, or combinationsthereof) along the length of their fluid flow path. The auxiliarychannel (202) can optionally possess one or more branch points. Theauxiliary channel (202) can further include additional elements, such aselectrodes, fluid inlets, fluid outlets, fluid reservoirs, valves,pumps, and combinations thereof, connected to the auxiliary channel tofacilitate device operation.

The auxiliary channel (202) can be electrochemically connected to thedesalination unit (100) via a bipolar electrode. In these embodiments,the bipolar electrode is configured so as to be in electrochemicalcontact with both the desalination unit (100) and the auxiliary channel(202), meaning that a first surface of the bipolar electrode canparticipate in a faradaic reaction with one or more components of asolution present in a microfluidic channel of the desalination unit, anda second surface of the bipolar electrode can participate in a faradaicreaction with one or more components of a solution present in theauxiliary channel. The device can be configured such that the bipolarelectrode comprises an anode in electrochemical contact with thedesalination unit and a cathode in electrochemical contact with theauxiliary channel during device operation. Alternatively, the device canbe configured such that the bipolar electrode comprises a cathode inelectrochemical contact with the desalination unit and an anode inelectrochemical contact with the auxiliary channel during deviceoperation.

By way of exemplification, referring again to the example deviceillustrated in FIG. 2, a bipolar electrode (204) electrochemicallyconnects the auxiliary channel (202) and the desalination unit (100). Afirst surface of the bipolar electrode (206) is in electrochemicalcontact with the desalination unit (100), and is positioned in proximityto the intersection (107). The first surface of the bipolar electrode(206) is configured to form an ion depletion zone (109) at anddownstream of the surface of the bipolar electrode during deviceoperation, resulting in the formation of an electric field gradient inproximity to the intersection. A second surface of the bipolar electrode(208) is in electrochemical contact with the auxiliary channel (202).

The first surface of the bipolar electrode (206) can occupy the sameposition within the desalination unit, and have the same dimensions asthe surface of electrode (108) described above with respect to the firstdesalination unit.

Referring again to FIG. 2, the example device further includes a fluidreservoir (110) fluidly connected to the upstream terminus of the inletchannel (102), a fluid reservoir (114) fluidly connected to thedownstream terminus of the dilute outlet channel (104), a fluidreservoir (112) fluidly connected to the downstream terminus of theconcentrated outlet channel (106), and fluid reservoirs (210 and 212)fluidly connected to the termini of the auxiliary channel (202).

A power supply can be configured to apply a potential bias across theauxiliary channel (202) and the desalination unit (100). A flow ofsaltwater (120) can be initiated from the inlet channel (102) to thedilute outlet channel (104) and the concentrated outlet channel (106).Upon application of a potential bias, an ion depletion zone (109) andsubsequent electric field gradient are formed near the first surface ofthe bipolar electrode (206) in proximity to the intersection (107). As aconsequence, ions in the saltwater are preferentially directed into theconcentrated outlet channel (106), resulting in a brine (122) flowingthrough the concentrated outlet channel. Desalted water (124) flows intothe dilute outlet channel (104).

In some embodiments, the auxiliary channel can comprise a desalinationunit. In these embodiments, the microfluidic device can comprise twodesalination units, which can be of identical or different structure. Anexample device comprising two desalination units is illustrated in FIG.3. The device includes a first desalination unit (100) electrochemicallyconnected to a second desalination unit (302) by a bipolar electrode(310). The first desalination unit (100) is fluidly isolated from thesecond desalination unit (302).

The first desalination unit (100) includes an inlet channel (102)fluidly connected to a dilute outlet channel (104) and a concentratedoutlet channel (106). The dilute outlet channel (104) and theconcentrated outlet channel (106) diverge from the inlet channel (102)at an intersection (107). The second desalination unit (302) includes aninlet channel (304) fluidly connected to a dilute outlet channel (306)and a concentrated outlet channel (308). The dilute outlet channel (306)and the concentrated outlet channel (308) diverge from the inlet channel(304) at an intersection (307).

A bipolar electrode (310) electrochemically connects the firstdesalination unit (100) and the second desalination unit (302). A firstsurface of the bipolar electrode (312) is in electrochemical contactwith the first desalination unit (100), and is positioned in proximityto the intersection (107). The first surface of the bipolar electrode(312) is configured to form an ion depletion zone (109) downstream ofthe surface of the bipolar electrode during device operation, resultingin the formation of an electric field gradient in proximity to theintersection of the first desalination unit. A second surface of thebipolar electrode (314) is in electrochemical contact with the seconddesalination unit (302), and is positioned in proximity to theintersection of the second desalination unit (307). The second surfaceof the bipolar electrode (314) is configured to form an ion depletionzone (309) downstream of the surface of the bipolar electrode duringdevice operation, resulting in the formation of an electric fieldgradient in proximity to the intersection of the second desalinationunit. The example device further includes fluid reservoirs (110 and 320)fluidly connected to the upstream termini of the inlet channels of thefirst and second desalination units, fluid reservoirs (114 and 322)fluidly connected to the downstream termini of the dilute outletchannels of the first and second desalination units, and fluidreservoirs (112 and 324) fluidly connected to the downstream termini ofthe concentrated outlet channels of the first and second desalinationunits.

The second desalination unit (302), as well as all of the elementsmaking up the second desalination unit (e.g., the inlet channel (304),the dilute outlet channel (306), and the concentrated outlet channel(308)) can have the same dimensions and relative configurations as thosedescribed above with respect to the first desalination unit. The firstsurface of the bipolar electrode (312) and the second surface of thebipolar electrode (314) can occupy the same positions within theirrespective desalination units, and have the same dimensions as thesurface of electrode (108) described above with respect to the firstdesalination unit.

A power supply can be configured to apply a potential bias across thefirst desalination unit (100) and the second desalination unit (302). Aflow of saltwater (120 and 330) can be initiated from the inlet channelsof the first and second desalination units to the dilute outlet channelsand the concentrated outlet channels of the first and seconddesalination units. Upon application of a potential bias, ion depletionzones (109 and 309) and subsequent electric field gradients are formednear the first surface of the bipolar electrode (312) in proximity tothe intersection (107) of the first desalination unit, and near thesecond surface of the bipolar electrode (314) in proximity to theintersection (307) of the second desalination unit. As a consequence,ions in the saltwater are preferentially directed into the concentratedoutlet channels of the first and second desalination units (106 and308), resulting in a brine (122 and 334) flowing through theconcentrated outlet channels of the first and second desalination units.Desalted water (124 and 332) flows into the dilute outlet channels (104and 306) of the first and second desalination units.

The microfluidic devices described herein can further include one ormore additional components (e.g., pressure gauges, valves, pressureinlets, pumps, fluid reservoirs, sensors, electrodes, power supplies,and combinations thereof) to facilitate device function. In someembodiments, the devices include a pump, valve, fluid reservoir, orcombination thereof configured to regulate fluid flow into the inletchannel of the device.

The devices can include a salinometer configured to measure the salinityof fluid flowing through one or more of the microfluidic channels of thedevice. For example, in some cases, the devices can include asalinometer configured to measure the salinity of fluid flowing throughthe dilute outlet channel. The salinometer can measure the salinity ofthe fluid via any suitable means. For example, the salinometer canmeasure the fluid's electrical conductivity, specific gravity, index ofrefraction, or combinations thereof.

In certain embodiments, the devices include a salinometer configured tomeasure the salinity of fluid flowing through the dilute outlet channel,and a pump, valve, fluid reservoir, or combination thereof configured toregulate fluid flow into the inlet channel of the device. The devicescan further include signal processing circuitry or a processorconfigured to operate the pump and/or valve connected to the inletchannel so as to adjust fluid flow into the inlet channel of the devicein response to the salinity of fluid flowing through the dilute outletchannel.

Systems

A plurality of the microfluidic devices described herein can be combinedto form a water purification system.

Water purification systems can comprise any number of the devicesdescribed herein. The number of devices incorporated within the waterpurification system can be selected in view of a number of factors,including the overall system design, the desired throughput of thesystem, salinity of the saltwater being treated using the system, andthe desired degree of salinity reduction.

In some cases, the inlet channels of two or more of the devices in thesystem are fluidly connected to a common water inlet, so as tofacilitate the flow of saltwater into the inlet channels of multipledevices in the system. Similarly, the dilute outlet channels of two ormore of the devices in the system can be fluidly connected to a commonwater outlet, so as to facilitate the collection of desalted water fromthe dilute outlet channels of multiple devices in the system.

The system can comprise a plurality of the devices described hereinarranged in parallel. Within the context of the systems describedherein, two devices can be described as being arranged in parallelwithin a system when fluid flowing from either the dilute outlet channelor the concentrated outlet channel of the first device in the systemdoes not subsequently flow into the inlet channel of the second devicein the system.

By way of example, FIG. 4 is a schematic drawing of a water purificationsystem (400) that includes a first desalination unit (402) and a seconddesalination unit (404) arranged in parallel. The example device furtherincludes an auxiliary channel (406) which is fluidly isolated from boththe first and second desalination unit. A first bipolar electrode (408)electrochemically connects the auxiliary channel (406) and the firstdesalination unit (402). A second bipolar electrode (410)electrochemically connects the auxiliary channel (406) and the seconddesalination unit (404). The example system can be operated by applyinga potential bias between the auxiliary channel and the first and seconddesalination units.

FIG. 5 illustrates a second example water purification system (500) thatincludes two devices arranged in parallel. The system (500) comprises afirst device which includes a first desalination unit (502)electrochemically connected to a first auxiliary channel (504) by afirst bipolar electrode (506). The system (500) further comprises asecond device which is arranged in parallel with respect to the firstdevice, and which includes a second desalination unit (508)electrochemically connected to a second auxiliary channel (510) by asecond bipolar electrode (512). As illustrated in FIG. 5, a power supplycan be configured to apply a potential bias across both the firstauxiliary channel (504) and desalination unit (502) and the secondauxiliary channel (510) and desalination unit (508).

The system can comprise a plurality of the devices described hereinfluidly connected in series. Within the context of the systems describedherein, two devices can be described as being fluidly connected inseries within a system when fluid flowing from either the dilute outletchannel or the concentrated outlet channel of the first device in thesystem subsequently flows into the inlet channel of the second device inthe system.

By way of example, FIG. 6 is a schematic drawing of a water purificationsystem (600) that includes two devices fluidly connected in series. Thesystem (600) includes a first desalination unit (602) and a seconddesalination unit (604) fluidly connected in series, such that thedilute outlet channel of the first desalination unit is fluidlyconnected to the inlet channel of the second desalination unit. Theexample device further includes an auxiliary channel (606) which isfluidly isolated from both the first and second desalination unit. Afirst bipolar electrode (608) electrochemically connects the auxiliarychannel (606) and the first desalination unit (602). A second bipolarelectrode (610) electrochemically connects the auxiliary channel (606)and the second desalination unit (604). The example system can beoperated by applying a potential bias between the auxiliary channel andthe first and second desalination units.

If desired, the systems can contain a plurality of devices both arrangedin parallel and fluidly connected in series. For example, the device caninclude a first pair devices fluidly connected in series which arearranged in parallel with a second pair of devices fluidly connected inseries.

Methods of Making

The microfluidic devices and systems described herein can be fabricatedfrom any substrate material which is non-conductive, and suitable forthe flow of aqueous solutions through the microfluidic channels of thedevice or system. For example, the device or system can be fabricated,in whole or in part, from glass, silicon, or combinations thereof. Thedevice or system can also be fabricated, in whole or in part, from apolymer and/or plastic, such as a polyester (e.g., polyethyleneterephthalate; PET) polyurethane, polycarbonate, halogenated polymer(e.g., polyvinyl chloride and/or fluorinated polymer such aspolytetrafluoroethylene (PTFE)), polyacrylate and/or poly methacrylate(e.g., polymethyl methacrylate; PMMA), silicone (e.g.,polydimethylsiloxane; PDMS), a thermosetting resin (e.g., Bakelite), ora copolymer, blend, and/or combination thereof. The device or system canalso be fabricated, in whole or in part, from a ceramic (e.g., siliconnitride, silicon carbide, titania, alumina, silica, etc.).

In certain embodiments, the device or system is fabricated, in whole orin part, from a photocurable epoxy. In certain embodiments, the deviceor system is fabricated, in whole or in part, from PDMS.

The microfluidic devices and systems described herein can be fabricatedusing a variety of microfabrication techniques known in the art.Suitable methods for the microfabrication of microfluidic devicesinclude, for example, lithography, etching, embossing, roll-to-rollmanufacturing, lamination, printing, and molding of polymericsubstrates. The microfabrication process can involve one or more of theprocesses described below (or similar processes). Different portions ofthe device or system can be fabricated using different methods, andsubsequently assembled or bonded together to form the final microfluidicdevice or system. Suitable fabrication methods can be selected in viewof a number of factors, including the nature of the substrate(s) used toform the device or system, performance requirements, and the dimensionsof the microfluidic features making up the device or system.

Lithography involves use of light or other form of energy such aselectron beam to selectively alter a substrate material. Typically, apolymeric material or precursor (e.g., photoresist, a light-resistantmaterial) is coated on a substrate and is selectively exposed to lightor other form of energy. Depending on the photoresist, exposed regionsof the photoresist either remain or are dissolved in subsequentprocessing steps known generally as “developing.” This process resultsin a pattern of the photoresist on the substrate. In some embodiments,the photoresist is used as a master in a molding process. In someembodiments, a polymeric precursor is poured on the substrate withphotoresist, polymerized (i.e., cured) and peeled off. The resultingpolymer is bonded or glued to another flat substrate after drillingholes for inlets and outlets.

In some embodiments, the photoresist is used as a mask for an etchingprocess. For example, after patterning photoresist on a siliconsubstrate, channels can be etched into the substrate using a deepreactive ion etch (DRIE) process or other chemical etching process knownin the art (e.g., plasma etch, KOH etch, HF etch, etc.). The photoresistcan then be removed, and the substrate can be bonded to anothersubstrate using one of any bonding procedures known in the art (e.g.,anodic bonding, adhesive bonding, direct bonding, eutectic bonding,etc.). Multiple lithographic and etching steps and machining steps suchas drilling can be included. Carbon electrodes may be fabricated inplace by means of photoresist pyrolysis.

In some embodiments, a polymeric substrate, such as PMMA, can be heatedand pressed against a master mold for an embossing process. The mastermold can be formed by a variety of processes, including lithography andmachining. The polymeric substrate can then be bonded with anothersubstrate to form a microfluidic device or system. Machining processescan be included if necessary.

Devices and systems can also be fabricated using an injection moldingprocess. In an injection molding process, a molten polymer or metal oralloy is injected into a suitable mold and allowed to cool and solidify.The mold typically consists of two parts that allow the molded componentto be removed. Parts thus manufactured can be bonded to result in thedevice or system.

In some embodiments, sacrificial etch can be used to form the device orsystem. Lithographic techniques can be used to pattern a material on asubstrate. This material can then be covered by another material ofdifferent chemical nature. This material can undergo lithography andetch processes, or another suitable machining process. The substrate canthen be exposed to a chemical agent that selectively removes the firstmaterial. In this way, channels can be formed in the second material,leaving voids where the first material was present before the etchprocess.

In some embodiments, microchannels can be directly machined into asubstrate by laser machining or CNC machining. If desired, severallayers can be machined, and subsequently bonded together to obtain thefinal device or system.

Electrodes as well as other electrical device components can befabricated within the devices and systems by patterning suitableconductive materials on and/or within substrate materials using a numberof suitable methods known in the art.

In one or more embodiments, the conductive material includes one or moremetals. Non-limiting examples of suitable metals include Sn, Zn, Au, Ag,Ni, Pt, Pd, Al, In, Cu, or a combination thereof. Other suitableconductive materials include metal oxides and conductive non-metals(e.g., carbon derivatives such as graphite). Conductive materials can bedeposited using a vacuum deposition process (e.g., cathodic arcdeposition, electron beam physical vapor deposition, evaporativedeposition, pulsed laser deposition, or sputter deposition). Conductivematerial can also be provided in the form of a conductive ink which canbe screen printed, ink-jet printed, or otherwise deposited onto thesurface of the substrate material to form an electrical devicecomponent. Conductive inks are typically formed by blending resins oradhesives with one or more powdered conductive materials such as Sn, Zn,Au, Ag, Ni, Pt, Pd, Al, In, Cu, graphite powder, carbon black, or otherconductive metals or metal alloys. Examples include carbon-based inks,silver inks, and aluminum inks.

When forming an electrical device component, such as an electrode, inthe devices or systems described herein, one or more conductivematerials will preferably be deposited or applied as a thin film. Incertain embodiments, the conductive layers are thin metallic or carbonfilms which are about 50 microns in thickness or less (e.g., about 40microns in thickness or less, about 30 microns in thickness or less,about 25 microns in thickness or less, about 20 microns in thickness orless, about 15 microns in thickness or less, about 10 microns inthickness or less, about 5 microns in thickness or less, about 1 micronin thickness or less, about 900 nm in thickness or less, about 800 nm inthickness or less, about 750 nm in thickness or less, about 700 nm inthickness or less, about 600 nm in thickness or less, about 500 nm inthickness or less, about 400 nm in thickness or less, about 300 nm inthickness or less, or about 250 nm in thickness or less).

Methods of Using

The microfluidic devices and systems described herein can be used todecrease the salinity of water. The salinity of water can be decreasedby flowing saltwater through the desalination unit of a device or systemdescribed herein, and performing a faradaic reaction at the electrodepositioned in proximity to the intersection of the desalination unit.The faradaic reaction generates an electric field gradient that directsions in the saltwater away from the dilute outlet channel of thedesalination unit, and towards the concentrated outlet channel of thedesalination unit. As a result, the salinity of water which flows intothe dilute outlet channel is lower than the salinity of the saltwaterflowing into the inlet channel.

In some embodiments, methods of decreasing the salinity of water includeproviding a flow of saltwater through the inlet channel of a devicedescribed herein or the water inlet of a system described herein,applying a potential bias to generate an electric field gradient thatinfluences the flow of ions in the saltwater through the desalinationunit of the device or the desalination units of the system, andcollecting water from the dilute outlet channel of the device or thewater outlet of the system. In these methods, the water collected fromthe dilute outlet channel of the device or the water outlet of thesystem can have a lower electrical conductivity than the saltwaterflowed through the inlet channel of the device or the water inlet of thesystem.

In some embodiments, the potential bias applied to generate an electricfield gradient is greater than about 1 volt (e.g., greater than about 2volts, greater than about 2.5 volts, greater than about 3 volts, greaterthan about 4 volts, greater than about 5 volts, greater than about 6volts, greater than about 7 volts, greater than about 8 volts, orgreater than about 9 volts). In some embodiments, the potential biasapplied to generate an electric field gradient is less than about 10volts (e.g., less than about 9 volts, less than about 9 volts, less thanabout 8 volts, less than about 7 volts, less than about 6 volts, lessthan about 5 volts, less than about 4 volts, less than about 3 volts,less than about 2.5 volts, or less than about 2 volts).

The potential bias applied to generate an electric field gradient canrange from any of the minimum voltages to any of the maximum voltagesdescribed above. In some embodiments, the potential bias applied togenerate an electric field gradient ranges from about 1 volt to about 10volts (e.g., from about 1 volt to about 7 volts, from about 2 volts toabout 7 volts, or from about 2.5 to about 5 volts).

In some embodiments, the flow rate of the saltwater through thedesalination unit of the device or the flow rate of the saltwaterthrough each desalination unit of the system ranges from about 0.01 toabout 1 microliter per minute (e.g., from about 0.05 to about 0.5microliters per minute, or from about 0.1 to about 0.5 microliters perminute). Suitable flow rates can be selected in view of a variety offactors including the architecture of the device or system, the salinityof the saltwater being treated using the device or system, and thedesired degree of salinity reduction.

The devices, systems, and methods described herein can be used todecrease the salinity of saltwater having any measurable concentrationof dissolved sodium chloride. The saltwater can be seawater (e.g.,saltwater having a conductivity of between about 4 S/m and about 6 S/m).The saltwater can be brackish water (e.g., saltwater having aconductivity of between about 0.05 S/m and about 4 S/m). In certainembodiments, the saltwater has a conductivity of greater than about 0.05S/m (e.g., greater than about 0.1 S/m, greater than about 0.5 S/m,greater than about 1.0 S/m, greater than about 2.0 S/m, greater thanabout 2.5 S/m, greater than about 3.0 S/m, greater than about 3.5 S/m,greater than about 4.0 S/m, greater than about 4.5 S/m, greater thanabout 5.0 S/m, or greater than about 5.5 S/m).

The devices, systems, and methods described herein can be used todecrease the salinity of saltwater by varying degrees. The degree ofsalinity reduction can depend on a number of factors, including thearchitecture of the device or system, and the salinity of the saltwaterbeing treated using the device or system.

In some embodiments, the conductivity of the water desalinated using thedevices, systems, and methods described herein (e.g., the watercollected from the dilute outlet channel of the device or the wateroutlet of the system) does not exceed about 90% of the conductivity ofthe saltwater flowed into the device or system (e.g., it does not exceedabout 80% of the conductivity of the saltwater flowed into the device orsystem, it does not exceed about 75% of the conductivity of thesaltwater flowed into the device or system, it does not exceed about 70%of the conductivity of the saltwater flowed into the device or system,it does not exceed about 60% of the conductivity of the saltwater flowedinto the device or system, it does not exceed about 50% of theconductivity of the saltwater flowed into the device or system, it doesnot exceed about 40% of the conductivity of the saltwater flowed intothe device or system, it does not exceed about 30% of the conductivityof the saltwater flowed into the device or system, it does not exceedabout 25% of the conductivity of the saltwater flowed into the device orsystem, it does not exceed about 20% of the conductivity of thesaltwater flowed into the device or system, it does not exceed about 10%of the conductivity of the saltwater flowed into the device or system,it does not exceed about 5% of the conductivity of the saltwater flowedinto the device or system, it does not exceed about 1% of theconductivity of the saltwater flowed into the device or system, it doesnot exceed about 0.5% of the conductivity of the saltwater flowed intothe device or system, it does not exceed about 0.1% of the conductivityof the saltwater flowed into the device or system, it does not exceedabout 0.05% of the conductivity of the saltwater flowed into the deviceor system, it does not exceed about 0.01% of the conductivity of thesaltwater flowed into the device or system, or less).

In some cases, water desalinated using the devices, systems, and methodsdescribed herein (e.g., water collected from the dilute outlet channelof the device or the water outlet of the system) has a conductivity ofless than about 2.0 S/m (e.g., less than about 1.75 S/m, less than about1.5 S/m, less than about 1.25 S/m, less than about 1.0 S/m, less thanabout 0.75 S/m, less than about 0.5 S/m, less than about 0.25 S/m, lessthan about 0.1 S/m, less than about 0.05 S/m, less than about 0.01 S/m,less than about 0.005 S/m, less than about 0.001 S/m, less than about5.0×10⁻⁴ S/m, less than about 1.0×10⁻⁴ S/m, less than about 5.0×10⁻⁵S/m, less than about 1.0×10⁻⁵ S/m, or less).

In some embodiments, the water desalinated using the devices, systems,and methods described herein (e.g., water collected from the diluteoutlet channel of the device or the water outlet of the system) isdrinking water (e.g., the water has a conductivity of from about 0.05S/m to about 0.005 S/m). In some embodiments, the water desalinatedusing the devices, systems, and methods described herein (e.g., watercollected from the dilute outlet channel of the device or the wateroutlet of the system) is ultrapure water (e.g., the water has aconductivity of from about 0.005 S/m to about 5.5×10⁻⁶ S/m).

If desired, water can be treated multiple times using the devices,systems, and methods described herein to achieve a desired decrease inthe salinity of the saltwater.

The devices and systems described herein can be used to desalinate waterwith greater energy efficiency than conventional desalination methods.In some cases, the devices and systems described herein can be used todesalinate water with at an energy efficiency of less than about 1000mWh/L (e.g., at least about 900 mWh/L, at least about 800 mWh/L, atleast about 750 mWh/L, at least about 700 mWh/L, at least about 600mWh/L, at least about 500 mWh/L, at least about 400 mWh/L, at leastabout 300 mWh/L, at least about 250 mWh/L, at least about 200 mWh/L, atleast about 100 mWh/L, at least about 90 mWh/L, at least about 80 mWh/L,at least about 75 mWh/L, at least about 70 mWh/L, at least about 60mWh/L, at least about 50 mWh/L, at least about 40 mWh/L, at least about30 mWh/L, at least about 25 mWh/L, at least about 20 mWh/L, at leastabout 15 mWh/L, or at least about 10 mWh/L, or at least about 5 mWh/L).In some embodiments, the devices and systems described herein can beused to desalinate water with at an energy efficiency ranging from anyof the minimum values above to about 1 mWh/L (e.g., from at least about1000 mWh/L to about 1 mWh/L, from at least about 500 mWh/L to about 1mWh/L, from at least about 100 mWh/L to about 1 mWh/L, from at leastabout 75 mWh/L to about 1 mWh/L, or from at least about 50 mWh/L toabout 1 mWh/L).

In some cases, the saltwater is not pre-treated prior to desalinationwith the devices and systems described herein. In other embodiments, thesaltwater can be treated prior to desalination. For example, the removalof multivalent cations (e.g., Ca²⁺, Mg²⁺, or combinations thereof) fromsaltwater prior to desalination could reduce precipitate formationwithin the device or system over long operation times. Accordingly, insome embodiments, the saltwater can be pre-treated to reduce the levelof dissolved multivalent cations in solution, for example, by contactingthe saltwater with a suitable ion exchange resin. If necessary,saltwater can also be pre-treated to remove debris, for example, bysedimentation and/or filtration. If desired, saltwater can also bedisinfected prior to desalination.

If desired for a particular end use, water can be further treatedfollowing desalination with the devices and systems described herein.For example, water can be fluoridated by addition of a suitable fluoridesalt, such as sodium fluoride, fluorosilicic acid, or sodiumfluorosilicate. Water can also be passed through an ion exchange resinand/or treated to adjust pH following desalination with the devices andsystems described herein.

EXAMPLES Example 1: Desalination Using a Microfluidic Device

A microelectrochemical cell comprising a desalination unit and anauxiliary channel spanned by a single bipolar electrode (BPE) was usedto desalinate seawater along a locally generated electric field gradientin the presence of pressure driven flow (PDF). Seawater desalination wasachieved by applying a potential bias between a parallel desalinationunit and auxiliary channel to drive the oxidation of chloride at theanodic pole of the bipolar electrode. At the cathodic pole, waterreduction occurs to support current flow.

The oxidation of chloride at the anodic pole of the BPE results in anion depletion zone and subsequent electric field gradient. The electricfield gradient directed ions flowing through the desalination unit intoa branching microchannel, creating a brine stream, while desalted watercontinued to flow forward when the rate of pressure driven flow wascontrolled. Seawater desalination could thus be achieved by controllingthe rate of pressure driven flow to create both a salted and desaltedstream.

Materials and Methods

Fabrication of Microfluidic Device

A PDMS/quartz hybrid microfluidic device was prepared usingmicrofabrication methods known in the art. The structure of themicrofluidic device is schematically illustrated in FIG. 2. The devicecomprises a desalination unit and an auxiliary channel spanned by asingle bipolar electrode.

A pyrolyzed photoresist carbon electrode was fabricated on a quartzslide (1 in×1 in). Photoresist was spin coated onto the slide at 3500rpm for 45 seconds, and then soft baked on a hot plate at 100° C. for 1minute to remove excess solvent. The device was then exposed to a UVlamp with patterned mask above to reveal the electrode (100 μm wide by6.3 mm long) design. The excess photoresist was then removed bydevelopment. The devices were then placed in a quartz tube furnace witha forming gas of 5% H₂ and 95% N₂ continuously flowing at 100 standardcubic centimeters per minute to allow the photoresist to pyrolyze. Afterpyrolysis, the device was cooled to room temperate.

A PDMS desalination unit (5.0 mm long and 22 μm tall) with a 100 μm wideinlet channel and 50 μm wide dilute outlet channel and concentratedoutlet channel was fabricated parallel to an auxiliary channel (5.0 mmlong, 22 μm tall, 100 μm wide) using a SU-8 photoresist mold patternedon a silicon wafer. The separation between the desalination unit and theauxiliary channel was 6.0 mm (center-to-center). The PDMS channels wererinsed with ethanol and dried under N₂, then the PDMS andquartz/electrode surfaces were exposed to an air plasma for 15 seconds,and finally the two parts were bound together with the BPE aligned atthe intersection where the dilute outlet channel and concentrated outletchannel diverge from the inlet channel. The PDMS/quartz microfluidicdevice was then placed in an oven at 65° C. for 5 min to promoteirreversible bonding.

Evaluation of Desalination

Seawater collected from Port Aransas, Tex. was used to evaluatedesalination. To prevent obstruction of the microfluidic channel, theseawater samples were allowed to undergo a simple sedimentation processbefore sample collection. The seawater was spiked with a cationic (20 μMRu(bpy)²⁺) tracer to fluorescently monitor the movement of ions throughthe desalination unit during desalination.

A solution height differential was created between the fluid reservoirfluidly connected to the inlet channel (110; V₁) and the fluidreservoirs fluidly connected to the concentrated outlet channel (112;V₂) and fluidly connected to the dilute outlet channel (114, V₃). Inthis way, a pressure driven flow (PDF) from right to left was initiated.

Results

Using Au driving electrodes, E_(tot)=2.5 V was applied to reservoirs 212and 210 while fluid reservoirs 110, 112, and 114 were grounded. Thepotential bias created a sufficiently large potential difference betweenthe poles of the BPE (204) to drive water oxidation and reduction at theBPE anode (206) and cathode (208). See Eqn. 1 and 2, respectively.Moreover, chloride oxidation occurred at the BPE anode (206; Eqn. 3)directly resulting in an ion depletion zone near the BPE as chlorine wasgenerated.

2H₂O−4e ⁻

O₂+4H⁺  (Eqn. 1)

2H₂O+2e ⁻

H₂+2OH⁻  (Eqn. 2)

2Cl⁻−2e ⁻

Cl₂(2)  (Eqn. 3)

In addition, H⁺ electrogenerated by water oxidation (Eqn. 1) canneutralize bicarbonate and borate that can be present in seawater,further contributing to the strength of the ion depletion zone (109) andsubsequently formed electric field gradient. With PDF from right toleft, seawater, and thus the ions present is seawater, were transportedtoward the electric field gradient formed at intersection where thedilute outlet channel (104) and concentrated outlet channel (106)diverge from the inlet channel (102).

The electrophoretic velocity (u_(ep)) of a charged analyte is governedby Eqn. 4, where μ_(ep) is the analyte's electrophoretic mobility and V₁is the local electric field strength.

u _(ep)=μ_(ep) V ₁  (Eqn. 4)

In all regions of the device depicted in FIG. 2, except near the iondepletion zone formed by the anode of the bipolar electrode in proximityto the intersection where dilute outlet channel and concentrated outletchannel diverge from the inlet channel, the transport of water and alldissolved species is controlled by PDF. As a consequence, all neutralsand ions to move generally in the direction of fluid flow (i.e., fromright to left) throughout the device. However, as ions approach thelocal electric field gradient formed by the electrode in proximity tothe intersection, they experience an increasing u_(ep) as the electricfield strength increases. In the case of cations, this gradient causesthem to redirect toward the grounded reservoir in the brine stream as aresult of the local electrophoretic velocity of the ions (u_(ep))exceeding the mean convective velocity of the fluid (PDF). To maintainelectroneutrality with the microchannel, anions are also redirected intothe brine stream.

The flow of ionic species through the microchannels of the device wasmonitored by observing the flow of Ru(bpy)²⁺ (a fluorescent cationictracer) through the device. FIGS. 7A and 7B are fluorescence micrographsof the device taken before (FIG. 7A) and after (FIG. 7B) application ofa potential bias. As shown in FIG. 7A, when no potential bias wasapplied, ions flowed through the inlet channel (702), and into both thedilute outlet channel (704) and the concentrated outlet channel (706).Upon application of a potential bias, an ion depletion zone andsubsequent electric field gradient are formed near the BPE anode (708)in proximity to the intersection (710) of the dilute outlet channel(704) and the concentrated outlet channel (706; FIG. 7B). As aconsequence, ions, including the fluorescent cationic tracer Ru(bpy)²⁺,are directed into the concentrated outlet channel (706). Desalted water(which is non-fluorescent in the micrograph due to the absence offluorescent cationic tracer Ru(bpy)²⁺) flows into the dilute outletchannel (704). These results demonstrate that both cations and anionsflow into the concentrated outlet channel (706). The initial applicationof 2.5 V creates an oxidizing environment near the BPE anode whichcauses partial dissolution of the Au anode.

To confirm that the formation of an ion depletion zone resulted in thedeionization of the fluid flowing into the device, a similar experimentwas conducted using a solution lacking chloride ions. If all chlorideions are eliminated from solution, one would not expect the BPE anode toinduce formation of an ion depletion zone and local electric fieldgradient (as in the case of saltwater containing chloride ions). In thecontrol experiment, a solution of Na₂SO₄ was flowed through the device.As shown in FIG. 8, upon application of 2.5 V, no decrease influorescence intensity near the BPE anode was observed. This finding wasconsistent with the seawater desalination being the result of an iondepletion zone formed near the BPE anode.

FIG. 9 shows a representative plot of total current flowing through thedevice (i_(tot)) vs. time. The steady-state operating current of thedevice was 65 nA. With a 2.5 V potential bias driving the desalinationprocess, the device operated at a power consumption of only 162.5 nW.

Fluid flow rates through the dilute outlet channel could be measuredusing non-charged beads. Fluid flow rates through the dilute outletchannel could also be measured by tracking the movement of fluorescenttracer after the 2.5 V driving potential was turned off, in which caseall mass transport was due to PDF.

The average operating fluid flow rate of the devices was ˜400 μm/s. Athigher fluid flow rates, the ion depletion zone does not extend as farinto the dilute outlet channel. Consequently, the desalination processbecomes less efficient, and ions begin to flow into the dilute outletchannel during device operation.

Using the device operating at 162.5 nW, 34 mWh/L energy efficiencieswere achieved. This energy efficiency is orders of magnitude higher thanthe current state-of-the-art seawater desalination technologies. Forexample, reverse osmosis is typically performed at energy efficienciesof approximately 5 Wh/L, and has only achieved maximum energyefficiencies of approximately 1.8 Wh/L. This superior efficiency of themicrofluidic device relative to reverse osmosis is particularly notablewhen considering that these reverse osmosis energy efficienciescorrespond to the efficiencies of industrial desalination facilities(which are often higher than efficiencies observed for the same processconducted on a smaller scale).

A reduction in device scale typically results in a decrease in energyefficiency. As a consequence, these devices appear to be extremelycompetitive for small-scale desalination use. Moreover, because littleequipment is required, and device operation only requires a 2.5 V powersupply, these devices can be used in water stresses regions. Inaddition, because BPEs do not require a direct electrical connection, itis possible to simultaneously operate numerous devices in parallel usinga simple power supply.

The devices, systems, and methods of the appended claims are not limitedin scope by the specific devices, systems, and methods described herein,which are intended as illustrations of a few aspects of the claims. Anydevices, systems, and methods that are functionally equivalent areintended to fall within the scope of the claims. Various modificationsof the devices, systems, and methods in addition to those shown anddescribed herein are intended to fall within the scope of the appendedclaims. Further, while only certain representative devices, systems, andmethod steps disclosed herein are specifically described, othercombinations of the devices, systems, and method steps also are intendedto fall within the scope of the appended claims, even if notspecifically recited. Thus, a combination of steps, elements,components, or constituents may be explicitly mentioned herein or less,however, other combinations of steps, elements, components, andconstituents are included, even though not explicitly stated.

The term “comprising” and variations thereof as used herein is usedsynonymously with the term “including” and variations thereof and areopen, non-limiting terms. Although the terms “comprising” and“including” have been used herein to describe various embodiments, theterms “consisting essentially of” and “consisting of” can be used inplace of “comprising” and “including” to provide for more specificembodiments of the invention and are also disclosed. Other than wherenoted, all numbers expressing geometries, dimensions, and so forth usedin the specification and claims are to be understood at the very least,and not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, to be construed in light of thenumber of significant digits and ordinary rounding approaches.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of skill in the artto which the disclosed invention belongs. Publications cited herein andthe materials for which they are cited are specifically incorporated byreference.

What is claimed is:
 1. A microfluidic device comprising (a) adesalination unit comprising an inlet channel fluidly connected to adilute outlet channel and a concentrated outlet channel, wherein thedilute outlet channel and the concentrated outlet channel diverge fromthe inlet channel at an intersection; and (b) an electrode inelectrochemical contact with the desalination unit; wherein theelectrode is configured to generate an electric field gradient inproximity to the intersection where the dilute outlet channel and theconcentrated outlet channel diverge from the inlet channel.
 2. Thedevice of claim 1, wherein the electrode comprises an anode.
 3. Thedevice of claim 1, wherein the inlet channel has a width of from about150 microns to about 25 microns.
 4. The device of claim 1, wherein thedilute outlet channel, the concentrated outlet channel, or both thedilute outlet channel and the concentrated outlet channel have a widthof from about 80 microns to about 10 microns.
 5. The device of claim 1,wherein the sum of the area of a cross-section of dilute outlet channeland the area of a cross-section of the concentrated outlet channel issubstantially equal to the area of a cross-section of the inlet channel.6. The device of claim 1, wherein the angle formed between the diluteoutlet channel and the concentrated outlet channel at the intersectionis 60 degrees or less.
 7. The device of claim 1, further comprising anauxiliary channel fluidly isolated from the desalination unit.
 8. Thedevice of claim 7, wherein the electrode comprises a bipolar electrodeelectrochemically connecting the desalination unit and the auxiliarychannel.
 9. The device of claim 8, wherein the bipolar electrodecomprises an anode in electrochemical contact with the desalination unitand a cathode in electrochemical contact with the auxiliary channel. 10.The device of claim 7, wherein the auxiliary channel comprises a seconddesalination unit comprising an inlet channel fluidly connected to adilute outlet channel and a concentrated outlet channel, wherein thedilute outlet channel and the concentrated outlet channel diverge fromthe inlet channel at an intersection; and an electrode inelectrochemical contact with the second desalination unit; wherein theelectrode is configured to generate an electric field gradient inproximity to the intersection where the dilute outlet channel and theconcentrated outlet channel diverge from the inlet channel.
 11. A waterpurification system comprising a plurality of devices defined by claim1, wherein the inlet channels of the plurality of devices are fluidlyconnected to a water inlet, and the dilute outlet channels of theplurality of devices are fluidly connected to a water outlet.
 12. Amethod of decreasing the salinity of water comprising (a) providing aflow of saltwater through the inlet channel of the device defined byclaim 1; (b) applying a potential bias to generate an electric fieldgradient that influences the flow of ions through the desalination unitof the device defined by claim 1; and (c) collecting water from thedilute outlet channel of the device defined by claim 1; wherein thewater collected from the dilute outlet channel of the device defined byclaim 1 has a lower electrical conductivity than the saltwater.
 13. Themethod of claim 14, wherein the saltwater comprises seawater.
 14. Themethod of claim 14, wherein the saltwater comprises brackish water. 15.The method of claim 14, wherein the conductivity of the water collecteddoes not exceed about 80% of the conductivity of the saltwater.
 16. Themethod of claim 14, wherein the water collected has a conductivity ofless than about 0.1 S/m.
 17. The method of claim 14, wherein the watercollected has a conductivity of from about 0.05 S/m to about 0.005 S/m18. The method of claim 14, wherein the water collected has aconductivity of from about 0.005 S/m to about 5.5×10⁻⁶ S/m.
 19. Themethod of claim 14, wherein potential bias applied ranges from about 1volt to about 10 volts.
 20. The method of claim 14, wherein the rate offlow of the saltwater through the desalination unit of the devicedefined by claim 1 ranges from about 0.01 to about 1 microliter perminute.
 21. A method of decreasing the salinity of water comprising (a)flowing saltwater through a desalination unit comprising an inletchannel fluidly connected to a dilute outlet channel and a concentratedoutlet channel, wherein the dilute outlet channel and concentratedoutlet channel diverge from the inlet channel at an intersection; and(b) performing a faradaic reaction at an electrode positioned inproximity to the intersection to generate an electric field gradient;wherein the electric field gradient directs ions in the saltwater awayfrom the dilute outlet channel.